The endothelium forms a semi-selective barrier between the blood and the interstitial space of all organs, including the lung. Disruption of this permeability barrier is a cardinal feature of inflammation that can lead to thrombus formation, vascular occlusion, tissue injury and shock. While it is clear that blood leukocytes stimulated during the inflammatory response are intricately involved in barrier disruption, the mechanisms responsible are not completely define. Thus, while oxidants and performed factors released by stimulated leukocytes are certainly important in the response, they cannot entirely account for the potent permeability- inducing effects exerted by stimulated neutrophils adhering to the vascular endothelium. We will employe a model system to assess the influence of phosphatIdic acid, a biologically active second messenger generated in plasma membranes of stimulated cells, on the integrity of the endothelial monolayer. We will investigate the interaction of phosphatidic acid with endothelial plasma membranes and define biochemical and functional responses of endothelial cells exposed to the phospholipid. We hope to define the role of a novel endothelial ecto-phosphatidic acid phosphohydrolase in regulating these response. We will explore the hypothesis that ligation of specific receptors for phosphatidic acid on the outer surface of the endothelial cell plasma membrane results in mobilization of intracellular stored calcium and resultant increases in permeability. We will investigate the possibility that these receptor sites are activated by phosphatidic acid generated within the plasma membrane of adherent cells, such as neutrophilic leukocytes, as they respond to inflammatory stimuli. Finally, we will attempt to identify the downstream effectors of phosphatidic acid-induced permeability. Based upon our preliminary studies, we will now focus on how MLCK-independent mechanisms mediate phosphatidic acid induction of permeability. The information gained as a result of this study will be essential in evaluating the potential role of phosphatidic acid in disruption of endothelial barrier function in biologic systems.